That might have been true a decade ago. But GPUs and FPGAs have long been obsolete for mining Bitcoin.
Mining is happening on custom silicon in large-scale operations. They specifically observed several of those large-scale operations in multiple nations and extrapolated out. I don't see how that methodology is flawed.
Bitcoin is deflationary. There is a hard limit on the total number of bitcoins that will ever exist. Every so often, the reward for mining a block is halved. Eventually there will be effectively zero reward for mining at all.